This invention relates to the field of microelectromechanical systems (MEMS) current carrying devices and power relays, and particularly to microelectromechanical current carrying devices and power relays with liquid metal contacts, such as mercury.
Electrical relays are extensively used in low voltage electric power distribution systems. As aircraft designs shift towards flight-by-wire and flight-by-light concepts, distributed power bus architectures are increasingly being adopted in newer aircraft and spacecraft. Under distributed power bus architecture, electric relays are replacing mechanical and pneumatic actuators, as the key components for power and signal distribution. Specifically in aerospace applications where radiation hardness (rad-hard) is an important consideration, MEMS based power relays offer significant advantages over solid state devices based on semiconductor p-n junctions. In general, power relays must have high current carrying capacity, low contact series impedance, fast switching operation, acceptable hold-off voltage, and they require sufficiently low control voltage.
Two of the main factors limiting the performance of MEMS based micro-relay devices have resulted from the use of high resistance thin metal layers to feed current to the contact region and the rapid contact degradation related to heat-enhanced electromigration. In general, devices using standard poly-silicon micromachining processes present high resistance in the metal-poly contact due to oxide buildup enhanced by local heating. An alternative approach is to use gold which has been demonstrated to perform better as a contact material since it does not oxidize and only requires the application of a small closing force for attaining a reliable contact. However, gold has the tendency to self-weld and electro-migration is still a problem.
Therefore, it is desirable to provide an improved microelectromechanical power relay.
It is also desirable to provide an improved microelectromechanical power relay capable of high power operation when configured in a stacked array.
It is also desirable generally to provide a means for carrying current using a liquid metal.
A microelectromechanical current carrying apparatus as disclosed herein comprises a microcavity chamber and a liquid metal filling the microcavity chamber. A voltage differential is applied between the liquid metal at lower and upper ends of this chamber, thereby causing a current to be carried by the liquid metal. In a preferred embodiment, lower and upper contacts contact the liquid metal at these lower and upper chamber ends for purpose of applying this voltage differential. To use this apparatus as a relay/switch, the upper contact is moved to establish and break the contact with the liquid metal at the upper end of the chamber to respectively initiate and terminate the current carriage between the lower and upper contacts. By having the upper contact reside in a default position where it is not in contact with the liquid metal, a control electrode may be activated to draw the upper contact away from its default position, toward the control electrode, and into contact with said liquid metal to initiate the current flow, and may further be deactivated to cease drawing the upper contact toward the control electrode, break the contact of the upper contact with the liquid metal to terminate the current flow, and allow the upper contact to return to its default position.
The present invention provides for a metal-mercury contact micro-relay based on silicon micromachining technology. When arranged in a parallel array of vertical micro-relays, the system is capable of switching currents on the order of 1 ampere per device array. Micromachined micro-relays can also function as mechanical switches, because they rely on majority carriers conduction and do not have any functional semiconductor junctions. They are inherently rad-hard devices suitable for use in space as a replacement for solid state devices and in other high radiation environment such as those found in the nuclear industry. Rapid switching of large current is a problem with solid contact based relays because of arcing when current flow is disrupted, causing damage to the contacts and degrading their conductivity due to pitting of the electrode surfaces. The liquid metal based MEMS relay eliminates the problem first by distributing the current between many relays in parallel to reduce the voltage on a single relay, and secondly because the contacts are liquid, they are self-healing.